It is often desirable to adjust the resistance value of a resistor to bring the resistance of the resistor to a desired value. For a film type resistor, i.e., a resistor having a film of a resistance material on the surface of a substrate and spaced terminations of a conductive material at the ends of the resistance film, this adjustment is generally achieved by changing the dimensions, i.e, the width and/or length, of the path of the resistance film between the terminations. This technique is referred to as "trimming" the resistance value of the resistor. One method of trimming the resistor is to cut a groove through the resistance film so as to change the width of the resistance. If the resistance film is on a cylindrical substrate, the groove can extend in a spiral path around the substrate. If the resistance film is on a flat surface of a substrate, the groove can extend from one edge of the resistance film or spaced grooves can be provided extending from opposite sides of the resistance path. This technique is shown in the U.S. Pat. No. 4,041,440, of James L. Davis et al, issued Aug. 9, 1977 and entitled "Method of Adjusting Resistance of a Thick-Film Thermistor".
A resistor whose resistance varies with changes in temperature, such as a thermistor, not only can require an adjustment of the resistance value of the resistor, but also an adjustment (trimming) of the temperature coefficient of resistance (TCR) of the resistor. Trimming the resistance value of the resistor by changing the length of the path of the resistor does not trim the TCR of the resistor. One method of adjusting the TCR of a temperature variable resistor is to change the composition of the thermistor material prior to making the thermistor. However, this will not compensate for variations in the geometry of the device after it is made. Therefore, it would be desirable to have a simple technique for trimming the TCR of a temperature variable resistor after the resistor is made.